If $6a + 7b = -5$ and $6x + 4y = 5$, what is $-28y + 42b + 36a - 42x$ ?
$= 36a + 42b - 42x - 28y$ $= (6) \cdot (6a + 7b) + (-7) \cdot (6x + 4y)$ $= (6) \cdot (-5) + (-7) \cdot (5)$ $= -30 - 35$ $= -65$